Depletion Interaction Research Articles

Rotational viscosity of nematic lyotropic chromonic liquid crystals

Lyotropic chromonic liquid crystal has great application potential in the areas of the biology, medicine, and optics due to its biocompatibility, nontoxicity, biodegradability and birefringence. The rotating magnetic field method was used to measure the rotational viscosity of nematic Sunset Yellow, disodium cromoglycate and Sunset Yellow doping of magnesium sulfate, sodium chloride and sodium hydroxide at various temperatures and concentrations. The rotational viscosity of nematic lyotropic chromonic liquid crystals is positively correlated with the square of the average length of columnar aggregates, increasing with the increase of concentration, but decreasing exponentially as temperature increases. This paper established a universal empirical expression of the rotational viscosity of nematic lyotropic chromonic liquid crystals and presented that the largest relative deviation between theoretical calculation and experimental results is only 22.60%. The rotational viscosity of nematic lyotropic chromonic liquid crystals becomes intensive with the increasing of magnesium sulfate and sodium chloride concentrations, while it reduces with the increase of sodium hydroxide concentration. The “one-step method” is proposed here that one sample of nematic LCLCs at various temperatures can replace normal materials to evaluate the influences of the shape anisotropy of molecules or colloidal particles on the research target, such as physical property parameters, depletion interaction potentials, and so on. This method is based on the self-assembly ability difference of lyotropic chromonic liquid crystal molecules at various temperatures and has been proved to be reliable by comparison with the results obtained from many samples of nematic 5CB, CBP5, 5PCB, et al.

Deep particle stocks following the summer bloom around the Kerguelen islands: Insights into diatoms physiological state, community structure and mortality modes

Particles located at the interface between the surface ocean layer and the top of the mesopelagic domain are the initial vector of the biological pump yet, their nature is still largely unknown. During the MOBYDICK cruise in the vicinity of the Kerguelen Islands (Indian sector of the Southern Ocean) we deployed a recently available device that concentrates and collects deep particles over a predetermined layer of water. In this paper, we present a detailed description of the collected particles and individual planktonic cells, including their taxonomy, carbon and lipid content, as well as cell viability, in order to characterize the particle stocks present at depth. The cruise was carried out at the end of the summer bloom, a period characterized by declining stocks of biogenic material associated with various mortality processes of planktonic organisms. Unexpectedly, the majority of the collected particles consisted of single empty diatom frustules, while fecal pellets and aggregates accounted for only a minor fraction. Distinct mortality processes, from parasitic infection to mesozooplankton grazing, and distinct silicification degrees as well as different life stages could be identified in relation to diatom taxa suggesting the occurrence of several export modes to intermediate and deep layers within the diatom community. We observed a dominant contribution of single cell diatoms (93 ± 6%) to the deep particle stocks but a very small contribution of intact diatom cells (~0.3%) to C content in the intermediate layer (125–500 m), together with a very small fecal pellet contribution, that was dominated by the minipellet size-class. Taxonomical analyses revealed distinct communities west of Kerguelen in the HNLC area compared to the island's fertilized plateau and its eastern flank. Differences in silicification degrees as well as distinct mortality/export processes linked to surface nutrient depletion and trophic interactions (such as parasitic infection or grazing by phaeodarians) were identified in the upper layer, leading to distinct contributions of major diatom taxa to deep suspended particles.

Rheological properties of phase transitions in polydisperse and monodisperse colloidal rod systems

AbstractRheological modifiers are added to formulations to tune rheology, enable function and drive phase changes requiring an understanding of material structure and properties. We characterize two colloidal rod systems during phase transitions using multiple particle tracking microrheology, which measures the Brownian motion of probes embedded in a sample. These systems include a colloid (monodisperse polyamide or polydisperse hydrogenated castor oil), surfactant (linear alkylbenzene sulfonate [LAS]), and nonabsorbing polymer (polyethylene oxide [PEO]) which drives gelation by depletion interactions. Phase transitions are characterized at all concentrations using time‐cure superposition. We determine that rheological evolution depends onLAS:colloid. The critical PEO concentration required to form a gel,cc/c*, is independent ofLAS:colloid, critical relaxation exponent,n, is dependent onLAS:colloid, and both are independent of colloid polydispersity.nindicates the material structure at the phase transition. AtLAS:colloid > 16, the scaffold is a tightly associated network and atLAS:colloid = 16 a loosely associated network.

Influence of solvent quality on depletion potentials in colloid-polymer mixtures.

As first explained by the classic Asakura-Oosawa (AO) model, effective attractive forces between colloidal particles induced by depletion of nonadsorbing polymers can drive demixing of colloid-polymer mixtures into colloid-rich and colloid-poor phases, with practical relevance for purification of water, stability of foods and pharmaceuticals, and macromolecular crowding in biological cells. By idealizing polymer coils as effective penetrable spheres, the AO model qualitatively captures the influence of polymer depletion on thermodynamic phase behavior of colloidal suspensions. In previous work, we extended the AO model to incorporate aspherical polymer conformations and showed that fluctuating shapes of random-walk coils can significantly modify depletion potentials [W. K. Lim and A. R. Denton, Soft Matter 12, 2247 (2016); J. Chem. Phys. 144, 024904 (2016)]. We further demonstrated that the shapes of polymers in crowded environments sensitively depend on solvent quality [W. J. Davis and A. R. Denton, J. Chem. Phys. 149, 124901 (2018)]. Here, we apply Monte Carlo simulation to analyze the influence of solvent quality on depletion potentials in mixtures of hard-sphere colloids and nonadsorbing polymer coils, modeled as ellipsoids whose principal radii fluctuate according to random-walk statistics. We consider both self-avoiding and non-self-avoiding random walks, corresponding to polymers in good and theta solvents, respectively. Our simulation results demonstrate that depletion of polymers of equal molecular weight induces much stronger attraction between colloids in good solvents than in theta solvents and confirm that depletion interactions are significantly influenced by aspherical polymer conformations.

From Attraction to Repulsion to Attraction: Non-monotonic Temperature Dependence of Polymer-Mediated Interactions in Colloidal Dispersions.

In this work, we have synthesized polystyrene particles that carry short end-grafted polyethylene glycol (PEG) chains. We then added dissolved 100 kDa PEG polymers and monitored potential flocculation by confocal microscopy. Qualitative predictions, based on previous theoretical developments in this field (Xie, F.; et al. Soft Matter 2016, 12, 658), suggest a non-monotonic temperature response. These theories propose that the "free" (dissolved) polymers will mediate attractive depletion interactions at room temperature, with a concomitant clustering/flocculation at a sufficiently high polymer concentration. At high temperatures, where the solvent is poorer, this is predicted to be replaced by attractive bridging interactions, again resulting in particle condensation. Interestingly enough, our theoretical framework, based on classical density functional theory, predicts an intermediate temperature regime where the polymer-mediated interactions are repulsive! This obviously implies a homogeneous dispersion in this regime. These qualitative predictions have been experimentally tested and confirmed in this work, where flocs of particles start to form at room temperature for a high enough polymer dosage. At temperatures near 45 °C, the flocs redisperse, and we obtain a homogeneous sample. However, samples at about 75 °C will again display clusters and eventually phase separation. Using results from these studies, we have been able to fine-tune parameters of our coarse-grained theoretical model, resulting in predictions of temperature-dependent stability that display semiquantitative accuracy. A crucial aspect is that under "intermediate" conditions, where the polymers neither adsorb nor desorb at the particle surfaces, the polymer-mediated equilibrium interaction is repulsive.

Aggregation of discoidal particles due to depletion interaction.

Depletion interactions between colloids of discoidal shape can induce their self-assembly into columnar aggregates. This is an effect of entropic origin with important implications in a range of colloidal systems, particularly in the clustering of erythrocytes that determine the rheological properties of blood. Here, we investigate the equilibrium state reached by discoidal colloids in a solution of smaller depletant particles. We develop a thermodynamic model of depletion-induced aggregation based on self-assembly theory and solve it analytically. We test the validity of the model by using Langevin simulations of a system of discs and depletant particles in which the depletion interaction emerges naturally. In addition, we consider the effect of an attractive interaction between depletant and discoidal particles, which we show induces a re-entrant dependence of aggregation with temperature.

Effective potentials induced by mixtures of patchy and hard co-solutes.

The addition of co-solutes to colloidal suspensions is often employed to induce tunable depletion interactions. In this work, we investigate effective colloidal interactions arising from binary co-solute mixtures of hard spheres and patchy particles. By changing the relative concentration of the two species, we show that the resulting effective potential Veff continuously changes from the one obtained for a single-component hard sphere co-solute to that mediated by the single-component patchy particle co-solute. Interestingly, we find that, independent of the relative concentration of the two components, the resulting Veff is additive, i.e., it is well-described by the linear combination of the effective interactions mediated by respective pure co-solutes. However, a breakdown of the additivity occurs when the co-solute mixture is close to the onset of a demixing transition. These results represent a step forward in understanding and predicting colloidal behavior in complex and crowded environments and for exploiting this knowledge to design targeted colloidal superstructures.

Homo-oligomerization of the human adenosine A2A receptor is driven by the intrinsically disordered C-terminus.

G protein-coupled receptors (GPCRs) have long been shown to exist as oligomers with functional properties distinct from those of the monomeric counterparts, but the driving factors of oligomerization remain relatively unexplored. Herein, we focus on the human adenosine A2A receptor (A2AR), a model GPCR that forms oligomers both in vitro and in vivo. Combining experimental and computational approaches, we discover that the intrinsically disordered C-terminus of A2AR drives receptor homo-oligomerization. The formation of A2AR oligomers declines progressively with the shortening of the C-terminus. Multiple interaction types are responsible for A2AR oligomerization, including disulfide linkages, hydrogen bonds, electrostatic interactions, and hydrophobic interactions. These interactions are enhanced by depletion interactions, giving rise to a tunable network of bonds that allow A2AR oligomers to adopt multiple interfaces. This study uncovers the disordered C-terminus as a prominent driving factor for the oligomerization of a GPCR, offering important insight into the effect of C-terminus modification on receptor oligomerization of A2AR and other GPCRs reconstituted in vitro for biophysical studies.

Effect of softness on glass melting and re-entrant solidification in mixtures of soft and hard colloids.

We present a systematic investigation of the structure and dynamic properties of model soft-hard colloidal mixtures. Results of a coarse-grained theoretical model are contrasted with rheological data, where the soft and hard colloids are mimicked by large star polymers with high functionality as the soft component and smaller stars with ultrahigh functionality as the hard one. Previous work by us revealed the recovery of the ergodicity of glassy soft star solutions and subsequent arrested phase separation and re-entrant solid transition upon progressive addition of small hard depletants. Here, we use different components to show that a small variation in softness has a significant impact on the state diagram of such mixtures. In particular, we establish that rendering the soft component more penetrable and modifying the size ratio bring about a remarkable shift in both the phase separation region and the glass-melting line so that the region of restored ergodicity can be notably enhanced and extended to much higher star polymer concentrations than for pure systems. We further rationalize our findings by analyzing the features of the depletion interaction induced by the smaller component that result from the interplay between the size ratio and the softness of the large component. These results demonstrate the great sensitivity of the phase behavior of entropic mixtures to small changes in the molecular architecture of the soft stars and point to the importance of accounting for details of the internal microstructure of soft colloidal particles for tailoring the flow properties of soft composites.

Competing interactions in the depletion forces of ternary colloidal mixtures

Depletion interactions between colloidal particles surrounded by smaller depletants are typically characterized by a strong attraction at contact and a moderately repulsive barrier in front of it that extends at distances similar to the size of the depletants; the appearance and height of the barrier basically depend on the concentration and, therefore, the correlation between depletants. From a thermodynamic point of view, the former can drive the system to phase separation or toward non-equilibrium states, such as gel-like states, but its effects on both local and global properties may be controlled by the latter, which acts as a kind of entropic gate. However, the latter has not been entirely analyzed and understood within the context of colloidal mixtures mainly driven by entropy. In this contribution, we present a systematic study of depletion forces in ternary mixtures of hard spherical particles with two species of depletants, in two and three dimensions. We focus the discussion on how the composition of the depletants becomes the main physical parameter that drives the competition between the attractive well and the repulsive barrier. Our results are obtained by means of the integral equation theory of depletion forces and techniques of contraction of the description adapted to molecular dynamics computer simulations.

Polymer brush-induced depletion interactions and clustering of membrane proteins

We investigate the effect of mobile polymer brushes on proteins embedded in biological membranes by employing both Asakura-Oosawa type of theoretical model and coarse-grained molecular dynamics simulations. The brush polymer-induced depletion attraction between proteins changes non-monotonically with the size of brush. The depletion interaction, which is determined by the ratio of the protein size to the grafting distance between brush polymers, increases linearly with the brush size as long as the polymer brush height is shorter than the protein size. When the brush height exceeds the protein size, however, the depletion attraction among proteins is slightly reduced. We also explore the possibility of the brush polymer-induced assembly of a large protein cluster, which can be related to one of many molecular mechanisms underlying recent experimental observations of integrin nanocluster formation and signaling.

Phase stability of colloidal mixtures of spheres and rods.

We determined the phase boundaries of aqueous mixtures containing colloidal rod-like fd-viruses and polystyrene spheres using diffusing-wave spectroscopy and compared the results with free volume theory predictions. Excluded volume interactions in mixtures of colloidal rods and spheres lead to mediated depletion interactions. The strength and range of this attractive interaction depend on the concentrations of the particles, the length L and diameter D of the rods, and the radius R of the spheres. At strong enough attraction, this depletion interaction leads to phase separation. We experimentally determined the rod and sphere concentrations where these phase transitions occur by systematically varying the size ratios L/R and D/R and the aspect ratio L/D. This was done by using spheres with different radii and modifying the effective diameter of the rods through either the ionic strength of the buffer or anchoring a polymeric brush to the surface of the rods. The observed phase transitions were from a binary fluid to a colloidal gas/liquid phase coexistence that occurred already at very low concentrations due to the depletion efficiency of highly anisotropic rods. The experimentally measured phase transitions were compared to phase boundaries obtained using free volume theory (FVT), a well established theory for calculating the phase behavior of colloidal particles mixed with depletants. We find good correspondence between the experimental phase transitions and the theoretical FVT model where the excluded volume of the rod-like depletants was explicitly accounted for in both the reservoir and the system.

Polymer Surfactant Interactions in Oil Enhanced Recovery Processes

A soil decontamination or enhanced oil recovery procedure typically requires the injection of a surfactant solution to reduce interfacial tension and promote oil recovery at the pore level, followed by the injection of a polymer solution to avoid the creation of preferential pathways and perform a homogeneous sweep of the reservoir. It is well known that polymers and surfactants interact with each other to form aggregates if they are of opposite charge or due to depletion interactions. To date, it is recommended to use polymers and surfactants that do not interact with each other to avoid clogging the wells. We show here that this precaution is not necessary and that in some cases, the use of interacting polymer and surfactant systems can even be an advantage for oil recovery. Contrary to previous studies, we injected previously mixed and homogeneous formulations into the porous medium and not a sequence of surfactant and polymer plugs. In addition to a total recovery of the oil in place, this strategy allows to limit the adsorption of the surfactant in the pore.

Topologically Protected Transport of Cargo in a Chiral Active Fluid Aided by Odd-Viscosity-Enhanced Depletion Interactions.

The discovery of topological edge states that unidirectionally propagate along the boundary of system without backscattering has enabled the development of new design principles for material or information transport. Here, we show that the topological edge flow supported by the chiral active fluid composed of spinners can even robustly transport an immersed intruder with the aid of the spinner-mediated depletion interaction between the intruder and boundary. Importantly, the effective interaction significantly depends on the dissipationless odd viscosity of the chiral active fluid, which originates from the spinning-induced breaking of time-reversal and parity symmetries, rendering the transport controllable. Our findings propose a novel avenue for robust cargo transport and could open a range of new possibilities throughout biological and microfluidic systems.

Phase separation in mixed suspensions of bacteria and nonadsorbing polymers.

The shapes of bacteria can vary widely; they may, for instance, be spherical, rod-like, string-like, or curved. In general, bacilli are highly anisotropic. For research and (bio)technological purposes, it can be useful to concentrate bacteria, which is possible by adding nonadsorbing polymers. The induced phase separation originates from a polymer-mediated depletion interaction, first understood by Asakura and Oosawa. Here, it is shown that free volume theory (FVT) can semi-quantitatively describe the phase transitions observed when adding sodium polystyrene sulfonate polymers to E. coli bacteria [Schwarz-Linek et al., Soft Matter 6, 4540 (2010)] at high ionic strength. The E. coli bacteria are described as short, hard spherocylinders. FVT predicts that the phase transitions of the mixtures result from a fluid-ABC crystal solid phase coexistence of a hard spherocylinder-polymer mixture.

Critical Depletion, Adsorption, and Intersurface Interaction in Polymer Solutions: A Mean-Field Theory Study

Using a self-consistent field theory, we examine the thermodynamics and interfacial properties of a homopolymer solution, either close to a single hard surface or confined between two parallel surfaces, in equilibrium with a bulk reservoir. We focus on the bulk critical regime and find that various physical quantities therein exhibit qualitatively different behaviors from those in the off-critical regime. For the single-surface system, the surface-excess Γ diverges as ln|ϕb – ϕc| and ln|χc – χ| when approaching the bulk critical point, where χ and ϕb are the Flory–Huggins interaction parameter and the volume fraction of polymer segment in bulk solution, respectively, and χc and ϕc are their respective values at the critical point. Γ diverges to +∞ when the surface–polymer interaction eₛ is larger than the depletion–adsorption transition (DAT) point ec, and to −∞ when eₛ < ec. As such, the DAT at ec in the critical regime is rather sharp and ec = 0.1824 + 0.1359N–¹/² at large N. For the two-surface system, the intersurface potential W at the critical point is purely attractive for both depleted and adsorbed cases and follows a D–³ power-law decay at large intersurface separations D; when eₛ is close to ec, the strength of W becomes rather weak. This nonmonotonic dependence of W on eₛ has important experimental implications on the stability of colloidal suspensions.

Impact of In-Cell and In-Vitro Crowding on the Conformations and Dynamics of an Intrinsically Disordered Protein.

The conformations and dynamics of proteins can be influenced by crowding from the large concentrations of macromolecules within cells. Intrinsically disordered proteins (IDPs) exhibit chain compaction in crowded solutions in vitro, but no such effects were observed in cultured mammalian cells. Here, to increase intracellular crowding, we reduced the cell volume by hyperosmotic stress and used an IDP as a crowding sensor for in-cell single-molecule spectroscopy. In these more crowded cells, the IDP exhibits compaction, slower chain dynamics, and much slower translational diffusion, indicating a pronounced concentration and length-scale dependence of crowding. In vitro, these effects cannot be reproduced with small but only with large polymeric crowders. The observations can be explained with polymer theory and depletion interactions and indicate that IDPs can diffuse much more efficiently through a crowded cytosol than a globular protein of similar dimensions.

Impact of In‐Cell and In‐Vitro Crowding on the Conformations and Dynamics of an Intrinsically Disordered Protein

AbstractThe conformations and dynamics of proteins can be influenced by crowding from the large concentrations of macromolecules within cells. Intrinsically disordered proteins (IDPs) exhibit chain compaction in crowded solutions in vitro, but no such effects were observed in cultured mammalian cells. Here, to increase intracellular crowding, we reduced the cell volume by hyperosmotic stress and used an IDP as a crowding sensor for in‐cell single‐molecule spectroscopy. In these more crowded cells, the IDP exhibits compaction, slower chain dynamics, and much slower translational diffusion, indicating a pronounced concentration and length‐scale dependence of crowding. In vitro, these effects cannot be reproduced with small but only with large polymeric crowders. The observations can be explained with polymer theory and depletion interactions and indicate that IDPs can diffuse much more efficiently through a crowded cytosol than a globular protein of similar dimensions.

Oligomerization of the Human Adenosine A2AReceptor is Driven by the Intrinsically Disordered C-Terminus

Truncation of intrinsically disordered C-terminal segments of G protein-coupled receptors (GPCRs) facilitates the biophysical studies of these important transmembrane protein receptors, but it is unclear how such truncation affects their structure and function. The human adenosine A2A receptor (A2AR) has an unusually long (∼120 amino acid residues) disordered C-terminus that is typically truncated prior to downstream structural and functional analyses. Herein, we demonstrate that the intrinsically disordered C-terminus of A2AR drives the oligomerization of this receptor, a process that is known to modulate the function of many GPCRs. We find that A2AR homo-oligomerization decreases progressively with the shortening of the C-terminus. Mutagenesis of 355ERR357 to 355AAA357 and of C394 as well as salting-out experiments revealed that the A2AR homo-oligomer interface consists of multiple interaction sites and types, including depletion interactions involving the C-terminus. This diversity of interactions sites and types give rise to multiple possible interfaces for the A2AR oligomers. Computational modeling confirms our experimental results by showing that the C-terminus of A2AR stabilizes the homo-oligomer interface via a diversity of inter-A2AR contacts. This study uncovers the disordered C-terminus of a GPCR as the driver of oligomerization, offering important guidance for in vitro structure-function studies of A2AR and other GPCRs.

Crowding-Induced Spatial Organization of Gene Expression in Cell-Sized Vesicles

E. coli cells, which are commonly used to study gene expression, show membraneless organization of gene expression components within the cell. Cell-free expression systems are also widely used to study gene expression, but a major limitation is the lack of a means to spatially organize DNA plasmids and ribosomes to mimic the cellular environment. In this work, we used computer simulations to guide experimental efforts to control the spatial organization of DNA and ribosomes in cell-sized vesicles using macromolecular crowding. Using Brownian dynamics simulations of coarse-grained models of DNA plasmids and crowders, we showed that plasmids remain uniformly distributed at low levels of crowding but become strongly adsorbed at confining surfaces at high levels of crowding. These effects are due to entropic depletion interactions resulting from the presence of crowding molecules. We validated the simulation results experimentally by using fluorescently-labelled DNA plasmids and ribosomes in cell-sized vesicles at different concentrations of the macromolecular crowder Ficoll-70. Large crowder concentrations resulted in preferential localization of plasmids at the walls of the vesicle, while ribosomes remained uniformly distributed throughout the vesicle. We then used kinetic Monte Carlo simulations to study how protein production was affected by crowding-induced changes in spatial organization and diffusion. The localization of DNA to the wall resulted in lower protein abundance and a decrease in translational efficiency with an increase in system size. Experimentally, we tracked the dynamics of transcription and translation in crowded vesicles using a coupled mRNA/protein reporter technique. These results were consistent with results from simulations. We thus used computer simulations to design a cell-free experimental platform capable of spatial organization of gene expression components. This platform can be used to better understand the spatial control of gene expression in cells.